Mechanisms For Forming Bonding Structures

ABSTRACT

Embodiments of mechanisms for forming a package are provided. The package includes a substrate and a contact pad formed on the substrate. The package also includes a conductive pillar bonded to the contact pad through solder formed between the conductive pillar and the contact pad. The solder is in direct contact with the conductive pillar.

This application is a divisional application of U.S. patent application Ser. No. 13/944,334, filed Jul. 17, 2013, entitled Mechanisms for Forming Bonding Structures,” which application is hereby incorporated herein in its entirety.

BACKGROUND

Semiconductor devices are used in a variety of electronic applications, such as personal computers, cell phones, digital cameras, or other electronic equipment. The semiconductor devices are fabricated by sequentially depositing insulating or dielectric layers, conductive layers, and semiconductor layers over a semiconductor substrate, and patterning the various material layers using lithography and etching processes to form circuit components and elements on the semiconductor substrate.

The semiconductor industry continues to improve the integration density of various electronic components (e.g., transistors, diodes, resistors, capacitors, etc.) by continual reductions in minimum feature size, which allow more components to be integrated into a given area. These smaller electronic components also require a smaller package that utilizes less area or a smaller height, in some applications.

New packaging technologies, such as package on package (PoP), have begun to be developed, in which a top package with a device die is bonded to a bottom package, with another device die. By adopting the new packaging technologies, the integration levels of the packages may be increased. These relatively new types of packaging technologies for semiconductor devices face manufacturing challenges.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompany drawings, in which:

FIG. 1 shows a perspective view of a package structure, in accordance with some embodiments.

FIGS. 2A and 2B are cross-sectional views of two packages before and after they are bonded to form a package structure, in accordance with some embodiments.

FIG. 2C is a cross-sectional view of two packages before they are bonded, in accordance with some embodiments.

FIGS. 3A and 3B are perspective views of stages of a process for arranging conductive pillars in a support substrate, in accordance with some embodiments.

FIGS. 3C-3E are cross-sectional views of various stages of a process for forming a package, in accordance with some embodiments.

FIGS. 4A-4E are cross-sectional views of portions of packages, in accordance with some embodiments.

FIGS. 5A-5C are cross-sectional views of portions of packages, in accordance with some embodiments.

FIGS. 6A-6D are cross-sectional views of portions of package structures, in accordance with some embodiments.

FIG. 7 is a cross-sectional view of a package, in accordance with some embodiments.

FIG. 8 is a cross-sectional view of a package structure, in accordance with some embodiments.

DETAILED DESCRIPTION OF THE ILLSSTRATIVE EMBODIMENTS

The making and using of the embodiments of the disclosure are discussed in detail below. It should be appreciated, however, that the embodiments can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative, and do not limit the scope of the disclosure.

It is to be understood that the following disclosure provides many different embodiments, or examples, for implementing different features of the disclosure. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. Moreover, the performance of a first process before a second process in the description that follows may include embodiments in which the second process is performed immediately after the first process, and may also include embodiments in which additional processes may be performed between the first and second processes. Various features may be arbitrarily drawn in different scales for the sake of simplicity and clarity. Furthermore, the formation of a first feature over or on a second feature in the description that follows include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact.

Some variations of the embodiments are described. Throughout the various views and illustrative embodiments, like reference numbers are used to designate like elements.

FIG. 1 shows a perspective view of a package structure 100 having a package 110 bonded to another package 120, which is further bonded to a substrate 130, in accordance with some embodiments. Package 110 is bonded to package 120 via bonding structures 115, and package 120 is bonded to substrate 130 via bonding structures 125. Each package, such as package 110 or package 120, includes one or more semiconductor dies. The semiconductor die includes a semiconductor substrate as used in semiconductor integrated circuit fabrication, and integrated circuits may be formed in and/or on the semiconductor substrate. The semiconductor substrate is defined to mean any construction including semiconductor materials, such as a bulk silicon, a semiconductor wafer, a silicon-on-insulator (SOI) substrate, or a silicon germanium substrate. Other semiconductor materials including group III, group IV, and group V elements may also be used.

The semiconductor substrate may further include isolation features (not shown), such as shallow trench isolation (STI) features or local oxidation of silicon (LOCOS) features. The isolation features may define and isolate various device elements. Examples of the various device elements that may be formed in the semiconductor substrate include transistors (e.g., metal oxide semiconductor field effect transistors (MOSFET), complementary metal oxide semiconductor (CMOS) transistors, bipolar junction transistors (BJT), high voltage transistors, high frequency transistors, p-channel and/or n-channel field effect transistors (PFETs/NFETs), etc.), diodes, or other suitable elements. Various processes are performed to form the various device elements including deposition, etching, implantation, photolithography, annealing, and/or other suitable processes. The device elements are interconnected to form the integrated circuit device, such as a logic device, memory device (e.g., SRAM), RF device, input/output (I/O) device, system-on-chip (SoC) device, combinations thereof, and other applicable types of devices.

Substrate 130 may be a semiconductor wafer, or a portion of a wafer. In some embodiments, substrate 130 includes silicon, gallium arsenide, silicon on insulator (“SOI”), or other similar materials. In some embodiments, substrate 130 also includes passive devices such as resistors, capacitors, inductors, and the like, or active devices such as transistors. In some embodiments, substrate 130 includes additional integrated circuits. Substrate 130 may further include through substrate vias (TSVs) and may be an interposer. In addition, substrate 130 may be made of other materials. In some embodiments, substrate 130 is a package substrate, such as a multiple-layer circuit board. In some embodiments, the package substrate also includes bismaleimide triazine (BT) resin, FR-4 (a composite material composed of woven fiberglass cloth with an epoxy resin binder that is flame resistant), ceramic, glass, plastic, tape, film, or other supporting materials that may carry conductive pads or lands needed to receive conductive terminals.

In some embodiments, each bonding structure 115 between packages 110 and 120 is formed by using a ball-to-ball bonding process. Two solder balls formed on opposite packages 110 and 120 are reflowed together to form one bonding structure 115. Similarly, bonding structures 125 between package 120 and substrate 130 may be formed by using the ball-to-ball bonding process described above.

Due to CTE (coefficient of thermal expansion) mismatch between packages 110 and 120, warpage of packages 110 and 120 may occur during a reflow process. To ensure that the solder balls on opposite packages 110 and 120 contact with each other to form bonding structures 115, large solder balls are used. As a result, spaces between adjacent bonding structures 115 are reduced, and the bridging risk is increased. In addition, during the ball-to-ball bonding process, the solder balls may slide and/or shift, which also leads to high bridging risk between bonding structures 115. Bonding structures 125 between package 120 and substrate 130 may also suffer from similar high bridging risk.

Bonding structures, such as bonding structures 115 or 125, may use through molding vias (TMVs) surrounded by a molding compound to reduce the bridging risk. However, the formation of TMVs involves forming openings in the molding compound by using a laser drilling process. Fabrication cost and time will increase with the number of TMVs. In addition, the pitch between bonding structures involving TMVs is high due to limitation imposed by the laser drilling process.

Therefore, it is desirable to find alternative mechanisms for forming bonding structures 115 or 125. FIGS. 2A and 2B are cross-sectional views of two packages before and after they are bonded to form a package structure 200, in accordance with some embodiments.

As shown in FIG. 2A, packages 110 and 120 are provided and ready for bonding, in accordance with some embodiments. In some embodiments, package 110 includes two semiconductor dies 224 and 226, and semiconductor die 224 is disposed over semiconductor die 226. However, package 110 could include a single semiconductor die or more than two semiconductor dies. In some embodiments, there is a glue layer (not shown) between semiconductor dies 224 and 226. Semiconductor dies 224 and 226 may include various device elements, such as memory devices.

Semiconductor die 226 is bonded to a substrate 216. Substrate 216 may be a semiconductor substrate including the various materials and/or components described above. Alternatively, substrate 216 may be a package substrate including the various materials described above. Semiconductor die 224 is electrically connected to conductive elements (not shown) formed on or in substrate 216 via bonding wires 228, in accordance with some embodiments. Similarly, semiconductor die 226 is electrically connected to the conductive elements formed on or in substrate 216 via bonding wires 230. Alternatively, semiconductor dies 224 and 226 are electrically connected to the conductive elements formed on or in substrate 216 via through substrate vias (TSVs) formed in semiconductor dies 224 and 226. Package 110 also includes a molding compound 232, which covers semiconductor dies 224 and 226 and bonding wires 228 and 230.

As shown in FIG. 2A, package 110 includes a passivation layer 220 formed on a bottom surface of substrate 216. Passivation layer 220 may include a solder resist layer, PBO layer, polyimide layer, epoxy layer, or other applicable dielectric layers. Passivation layer 220 has openings which expose contact pads 218 formed over the bottom surface of substrate 216. Contact pads 218 may be electrically connected to interconnect structures in substrate 216, and the interconnect structures may be further connected to devices in semiconductor dies 224 and 226 through bonding wires 228 and 230. In some embodiments, passivation layer 220 is not needed. Package 110 also includes a number of solder bumps 221 formed on contact pads 218 which are exposed.

As shown in FIG. 2A, package 120 includes a semiconductor die 208 bonded to a substrate 202. Connectors 210 are formed between semiconductor die 208 and substrate 202 to electrically connect semiconductor die 208 with conductive elements (not shown) formed on or in substrate 202. Substrate 202 may be a semiconductor substrate including the various materials and/or components described above. Alternatively, substrate 202 may be a package substrate including the various materials described above.

As shown in FIG. 2A, a number of contact pads 206 are formed on an upper surface of substrate 202. A passivation layer 204 is deposited and patterned over substrate 202. Passivation layer 204 has openings which expose portions of contact pads 206. Contact pads 206 may electrically connect to interconnect structures in substrate 202 and passivation layer 204 and therefore in communicate with semiconductor die 208 through connectors 210. In some embodiments, a number of conductive connectors 201 are formed on a bottom surface of substrate 202. Conductive connectors 201 are used to electrically connect to other conductive elements formed on another substrate, such as substrate 130 shown in FIG. 1. Therefore, conductive pads 206 may electrically connect to the interconnect structures in substrate 202 and in communicate with another substrate through conductive connectors 201. In some embodiments, passivation layer 204 is not needed.

As shown in FIG. 2A, a number of conductive pillars 214 are attached to contact pads 206 through solder 212, in accordance with some embodiments. In some embodiments, conductive pillars 214 are placed on solder 212 applied on contact pads 206. Afterwards, a reflow process is performed to bond conductive pillar 214 on contact pads 206 by solder 212 between conductive pillar 214 and contact pads 206. Conductive pillars 214 are secured on contact pads 206 after the reflow process. As shown in FIG. 2A, package 110 is positioned above and aligned with package 120 such that solder bumps 221 are aligned with conductive pillars 214.

In some embodiments, conductive pillars 214 are made of Cu, Al, Cu alloy, Al alloy, Au, other applicable materials, or combinations thereof. In some embodiments, each conductive pillar 214 has a height in a range from about 100 μm to about 300 μm. In some embodiments, each conductive pillar 214 has a diameter in a range from about 50 μm to about 200 μm.

As shown in FIG. 2B, packages 110 and 120 are bonded through bonding structures 215 to form package structure 200, in accordance with some embodiments. Package 110 is placed over package 120 and pressed to package 120 during a second reflow process. During the second reflow process, solder bumps 221 are reflowed to correspondingly cover conductive pillars 214, and therefore forming bonding structures 215. Each bonding structure 215 includes contact pad 218, a solder element 242, conductive pillar 214, and contact pad 206. Solder element 242 is formed by reflowing solder 212 and solder bump 221 shown in FIG. 2A.

As shown in FIG. 2B, conductive pillars 214 surrounded by solder element 242 are tall and slender, and therefore pitch P between bonding structures 215 (e.g. between conductive pillars 214) is reduced. In some embodiments, pitch P is in a range from about 150 μm to about 500 μm. In addition, such bonding processes reduce the sliding and shifting problems.

Alternatively, conductive pillars 214′ may be attached to package 110. FIG. 2C is a cross-sectional view of two packages before they are bonded, in accordance with some embodiments. Conductive pillars are not limited to be formed on the bottom package (package 120). As shown in FIG. 2C, conductive pillars 214′ are attached to contact pads 218 of package 110 through solder 212′. Package 110 is positioned above and aligned with package 120. Conductive pillars 214′ are aligned with solder bumps 221′ formed on contact pads 206 of package 120. After the alignment is performed, a process similar to that described in FIG. 2B may be performed to form a package structure similar to package structure 200 shown in FIG. 2B.

Packages having conductive pillars may be formed by using a variety of processes. FIGS. 3A and 3B are perspective views of stages of a process for arranging conductive pillars 214 in a support substrate 302, in accordance with some embodiments. FIGS. 3C-3E are cross-sectional views of various stages of a process for forming package 120, in accordance with some embodiments.

As shown in FIG. 3A, a number of conductive pillars 214 are spread out on support substrate 302 having multiple cavities 304, in accordance with some embodiments. In some embodiments, conductive pillars 214 have substantially planar top surfaces. In some other embodiments, conductive pillars 214 have curved top surfaces. Conductive pillars 214 may be formed by cutting a conductive wire, such as a Cu wire. Support substrate 302 may be made of bakelite, plastic steel, metal, or other applicable materials. Each cavity 304 of support substrate 302 has a diameter similar to or slightly larger than that of each conductive pillar 214. Cavities 304 may be formed by using a laser drilling process, mechanical drilling process, etching process, or other applicable processes. The number of conductive pillars 214 is larger than or equal to the number of cavities 304.

In some embodiments, support substrate 302 is vibrated by using an agitation generator (not shown) such that conductive pillars 214 fall into cavities 304, respectively. After each cavity 304 contains one conductive pillar 214, excess conductive pillars 214 outside of cavities 304 are removed, as shown in FIG. 3B. In some embodiments, each conductive pillar 214 is lodged in one of cavities 304. In some embodiments, a vacuum system (not shown in FIG. 3B) is attached to the backside of support substrate 302 to keep conductive pillars 214 stay in cavities 304. In some embodiments, the vacuum system is turned on when support substrate 302 is vibrated to allow conductive pillars 214 falling into cavities 304.

In some embodiments, conductive pillars 214 are made of Cu, and a protection layer is coated on conductive pillars 214. For example, the protection layer is coated on conductive pillars 214 after each conductive pillar 214 is located in one cavity 304. In some other embodiments, the protection layer is coated on conductive pillars 214 before they are spread out on support substrate 302. The protection layer may include an Ni layer, Ag layer, Ti layer, another applicable layer, or combinations thereof. The protection layer may prevent conductive pillars 214 from being oxidized.

As shown in FIG. 3C, support substrate 302 has holes 306 connected with cavities 304, in accordance with some embodiments. In some embodiments, holes 306 are connected with a vacuum chamber 308, and vacuum chamber 308 is further connected to a vacuum system 312. Vacuum system 312 is used to secure conductive pillars 214 through holes 306. However, in some other embodiments, holes 306, vacuum chamber 308, and vacuum system 312 are not required. As shown in FIG. 3C, support substrate 302 is positioned over substrate 202, such that conductive pillars 214 are aligned with solder 212 applied over conductive pads 206.

As shown in FIG. 3D, after the alignment is performed, conductive pillars 214 are placed on solder 212 on contact pads 206, in accordance with some embodiments. Afterwards, vacuum system 312 is turned off. After vacuum system 312 is turned off, a reflow process 310 is performed to reflow solder 212 such that conductive pillars 214 are bonded to contact pads 206. Support substrate 302 is used to hold conductive pillars 214 during reflow process 310.

Each conductive pillar 214 has a width W_(c), and each contact pad 206 has a width W_(p). In some embodiments, a ratio of width W_(c) to width W_(p) is smaller than ½, such as in a range of about 0.2 to about 0.49. Support substrate 302 is used to hold conductive pillars 214 during reflow process 310. Support substrate 302 prevents conductive pillars 214 from collapsing during reflow process 310.

In some embodiments, the ratio of width W_(c) to width W_(p) is larger than ½, such as in a range of about 0.51 to about 1.2. Since width W_(c) is relatively large, each conductive pillar 214 may not easily collapse during reflow process 310. Therefore, support substrate 302 is not needed to hold conductive pillars 214 during reflow process 310. However, in some embodiments, support substrate 302 is still used to hold conductive pillars 214 during reflow process 310 even if the ratio of width W_(c) to width W_(p) is larger than ½.

After reflow process 310, support substrate 302 is removed, and package 120 is formed, as shown in FIG. 3E. Package 120 includes conductive pillars 214 bonded to contact pads 206 through solder 212 between conductive pillars 214 and contact pads 206. In some embodiments, solder 212 is in direct contact with one of conductive pillars 214 and one of contact pads 206, while conductive pillars 214 are not in direct contact with contact pads 206. After package 120 is formed, package structure 120 is bonded to package 110 through bonding structures 215, as shown in FIGS. 2A-2B.

Package 120 having conductive pillars 214 has many variations. FIGS. 4A-4E are cross-sectional views of portions of packages 120, in accordance with some embodiments.

In some embodiments, conductive pillar 214 has a single width, and the ratio of width of conductive pillar 214 to width of contact pad 206 is variable. As shown in FIG. 4A, a ratio of a width W_(ca) of a conductive pillar 214 a to width W_(p) of contact pad 206 is smaller than ½, in accordance with some embodiments. The ratio of width W_(ca) of conductive pillar 214 a to width W_(p) of contact pad 206 may be in a range from about 0.2 to about 0.49. As shown in FIG. 4B, a ratio of a width W_(cb) of a conductive pillar 214 b to width W_(p) is larger than ½ but smaller than 1, in accordance with some embodiments. The ratio of width W_(cb) of conductive pillar 214 b to width W_(p) of contact pad 206 may be in a range from about 0.51 to about 0.9. As shown in FIG. 4C, a ratio of width W_(cc) of a conductive pillar 214 c to width W_(p) is larger than 1, in accordance with some embodiments. The ratio of width W_(cc) of conductive pillar 214 c to width W_(p) of contact pad 206 may be in a range from about 1.1 to about 1.5.

In some other embodiments, conductive pillar 214 has a narrow top portion and a wide bottom portion. The top portion has a width larger than that of the bottom portion. In various embodiments, a ratio of the width of the wide bottom portion to width W_(p) of contact pad 206 is variable.

As shown in FIG. 4D, a conductive pillar 214 d has two different widths W_(wd) and W_(nd). Width W_(wd) of bottom portion 315 b of conductive pillar 214 d is smaller than width W_(p) of contact pad 206, in accordance with some embodiments. A ratio of width W_(wd) to width W_(p) is larger than ½, such that bottom portion 315 b of conductive pillar 214 d may function as a support base of conductive pillar 214 d. The ratio of width W_(wd) of bottom portion 315 b of conductive pillar 214 d to width W_(p) of contact pad 206 may be in a range from about 0.51 to about 0.9. The ratio of width W_(nd) to width W_(p) may be in a range from about 0.1 to about 0.4.

As shown in FIG. 4E, width W_(we) of bottom portion 315 b of a conductive pillar 214 e is larger than width W_(p) of contact pad 206, in accordance with some embodiments. The ratio of width W_(we) of bottom portion 315 b of conductive pillar 214 e to width W_(p) of contact pad 206 may be in a range from about 1.1 to about 1.5. The ratio of width W_(ne) to width W_(p) may be in a range from about 0.1 to about 1.

Embodiments of the disclosure have many variations. For example, the amount of solder 212 is variable. FIG. 5A shows a structure similar to that shown in FIG. 4A except solder 212 having a larger amount (volume) than that shown in FIG. 4A. In some embodiments, solder 212 covers a center point C of the sidewall surface of conductive pillar 214 a. As shown in FIG. 5A, the top of solder 212 is higher than center point C.

FIG. 5B shows a structure similar to that shown in FIG. 4B except solder 212 having a larger amount (volume) than that shown in FIG. 4B. As shown in FIG. 5B, a larger amount (volume) of solder 212 is used, and solder 212 covers center point C of the sidewall surface of conductive pillar 214 b. As shown in FIG. 5B, the top of solder 212 is higher than center point C.

FIG. 5C shows a structure similar to that shown in FIG. 4D except solder 212 having a larger amount (volume) than that shown in FIG. 4D. As shown in FIG. 5C, solder 212 covers a center point C′ of a sidewall surface of upper portion 315 a of conductive pillar 214 d. As shown in FIG. 5C, the top of solder 212 is higher than center point C′.

As mentioned above, package 120 having conductive pillars 214 has many variations. In addition, package structure 200 having bonding structure 215 including conductive pillars 214 also has many variations. FIGS. 6A-6D are cross-sectional views of portions of package structures 200, in accordance with some embodiments.

In some embodiments, package 120 shown in FIG. 4A is bonded to package 110 (see FIG. 2A) to form package structure 200, as shown in FIG. 6A. Solder 212 and solder ball 221 are reflowed together to form solder element 242, and bonding structure 215 is formed. In some embodiments, solder element 242 covers sidewall surfaces of conductive pillar 214 a. In some embodiments, solder element 242 covers the entire surface of conductive pillar 214 a.

As shown in FIG. 6A, since solder 212 and solder ball 221 are reflowed and elongated to form solder element 242, the stress in solder element 242 is redistributed. If packages 110 and 120 are bonded by a round solder bump having a ball shape, high stress may concentrate at corner regions of the bonding structure, resulting in bump cracking and reduce the yield of package structure. However, the stress in elongated solder element 242 is redistributed, and therefore the corner regions of elongated solder element 242 suffer from less stress than the regular round solder bump.

It is noted that although a width of solder element 242 at a central portion of conductive pillar 214 a is smaller than that at end portions of conductive pillar 214 a, solder element 242 may be in other shapes. For example, the width of solder element 242 at the central portion of conductive pillar 214 a may be the same with or slightly larger than that at the end portions of conductive pillar 214 a.

Instead of conductive pillar 214 a, conductive pillar 214 b in FIG. 4B may be used to be bonded to package 110, and solder element 242 may also cover sidewall surfaces of conductive pillar 214 b (not shown).

In some embodiments, package 120 shown in FIG. 4C is bonded to package 110 (see FIG. 2A) to form package structure 200, as shown in FIG. 6B. Conductive pillar 214 c is pressed to solder bump 221 and the second reflow process is performed. After the second reflow process, solder bump 221 is reflowed to form solder 222 by which conductive pillar 214 and contact pad 218 are bonded, and bonding structure 215 is formed. In some embodiments, the sidewall surface of conductive pillar 214 c is not covered by any solder.

In some embodiments, package 120 shown in FIG. 4D is bonded to package 110 (see FIG. 2A) to form package structure 200, as shown in FIG. 6C. Solder 212 and solder bump 221 are reflowed together to form solder element 242. A bonding structure 215′ is therefore formed. Bonding structure 215′ includes contact pad 218, solder element 242, conductive pillar 214 d, and contact pad 206. In some embodiments, solder element 242 covers sidewall surfaces of conductive pillar 214 d. In some embodiments, solder element 242 covers the entire surface of conductive pillar 214 d.

In some embodiments, package 120 shown in FIG. 4E is bonded to package 110 (see FIG. 2A) to form package structure 200, as shown in FIG. 6D. Conductive pillar 214 e is pressed into solder bump 221 during the second reflow process is performed. After the second reflow process, solder bump 221 is reflowed to form solder 222 by which conductive pillar 214 e and contact pad 218 are bonded. Bonding structure 215′ is therefore formed.

In some embodiments, various conductive pillars are used in package 120. FIG. 7 is a cross-sectional view of package 120 having two or more different shapes of conductive pillars, such as conductive pillars 214 a and 214 d, in accordance with some embodiments. As shown in FIG. 7, conductive pillar 214 a has a single width W_(c), and conductive pillar 214 d has two different widths W_(n) and W_(w). Conductive pillars 214 a and 214 d are both bonded to contact pads 206 by solder 212. In some embodiments, a support substrate (not shown), having both large cavities and small cavities, is used to secure conductive pillars 214 a and 214 d. In some embodiments, conductive pillars 214 a are first secured in the large cavities in the support substrate, and conductive pillars 214 d are secured in the small cavities in the support substrate afterwards. Processes similar to the embodiments described in FIGS. 3A-3E are performed, and package 120 having both conductive pillars 214 a and 214 d are formed.

In some embodiments, the melting point of solder bump 221 of package 110 is higher than the melting point of solder 212. Therefore, when the second reflow process is performed to bond conductive pillar 214 with contact pad 218, solder 212 may melt before solder bump 221 does. As a result, conductive pillar 214 collapses during the second reflow process, and yield of package structure 200 is reduced.

In order to reduce or resolve the problem mentioned above, a protective layer 702 is formed to prevent conductive pillars 214 from collapsing. FIG. 8 is a cross-sectional view of package 200 having protective layer 702, in accordance with some embodiments.

As shown in FIG. 8, before packages 110 and 120 are bonded through bonding structures 215 and 215′, protective layer 702 is formed over package 120. Protective layer 702 may be a molding compound. Protective layer 702 has a height H_(M), which is smaller than a height H_(c) of conductive pillar 214 a or a height H_(p) of conductive pillar 214 d. Protective layer 702 covers solder 212 and a portion of conductive pillar 214 a and a portion of conductive pillar 214 d. Even if the second reflow process is performed, solder 212 which melts, conductive pillar 214, and conductive pillar 214 can be held by protective layer 702. Therefore, conductive pillar 214 a and conductive pillar 214 d are prevented from collapsing. The yield of package structure 200 is significantly increased.

In some other embodiments, the melting point T₁ of solder 212 is higher than the melting point T₂ of solder bump 221 (or that of solder 222). Therefore, protective layer 702 may not be needed since solder 212 may not melt during the second reflow process. In some embodiments, the melting point T₁ in a range from about 200□ to about 220□, and the melting points T₂ is in a range from about 220□ to about 270□. In some embodiments, the melting point T₁ is higher than the melting point T₂ by a temperature difference ΔT. The temperature difference ΔT may be in a range from about 20□ to about 50□.

In some embodiments, conductive pillars 214, such as conductive pillar 214 c shown in FIG. 6B, is wider than contact pad 206. In these cases, protective layer 702 may not be needed. In some other embodiments, bottom portion of conductive pillars 214, such as conductive pillar 214 e shown in FIG. 6D, is wider than contact pad 206. In these cases, protective layer 702 may not be needed.

In some embodiments, height H_(c) of conductive pillar 214 is smaller than width W_(c) of conductive pillar 214. In these cases, protective layer 702 may not be needed. In some embodiments, height H_(p) of conductive pillar 214 is smaller than width W_(w) of conductive pillar 214. In these cases, protective layer 702 may not be needed.

As described above, conductive pillars 214, such as conductive pillars 214 a, 214 b, 214 c, 214 d, and 214 e, are attached to contact pads 206 of package 120 by solder 212. Therefore, conductive pillars 214 are directly disposed on solder 212 (instead of being formed on contact pads 206 by plating) and are not in direct contact with contact pads 206. Therefore, the manufacturing process is simple and low-cost. In addition, since conductive pillars 214 are tall and slender, pitch P between bonding structures 215 is greatly reduced. Sliding and shifting, which usually occurs in a ball-to-ball bonding process, are prevented. Therefore, the yield of the package is greatly improved.

Embodiments of mechanisms for forming a bonding structure(s) between die packages are provided. The bonding structures with conductive pillars enable the reduction of the pitch between the bonding structures. In addition, manufacturing process of the bonding structures is relatively low-cost. Various embodiments of the conductive pillars are also described.

In accordance with some embodiments, a package is provided. The package includes a substrate and a contact pad formed on the substrate. The package also includes a conductive pillar bonded to the contact pad through solder formed between the conductive pillar and the contact pad. The solder is in direct contact with the conductive pillar.

In accordance with some embodiments, a package structure is provided. The pacakge structure includes a substrate and a second substrate bonded to the substrate through a bonding structure. The bonding structure includes a first contact pad formed on the substrate and a second contact pad formed on the second substrate. The bonding structure also includes a conductive pillar bonded to the first contact pad and the second contact pad through a solder element. The solder element is in direct contact with the conductive pillar.

In accordance with some embodiments, a method for forming a package structure is provided. The method includes providing a substrate having contact pads formed on the substrate and applying solder on the contact pads. The method also includes attaching conductive pillars on the contact pads by solder between the conductive pads and the conductive pillars. The method also includes providing a second substrate with solder bumps. The method further includes bonding the solder bumps and the conductive pillars to form bonding structures between the substrate and the second substrate.

Although the embodiments and their advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made herein without departing from the spirit and scope of the embodiments as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods, and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. In addition, each claim constitutes a separate embodiment, and the combination of various claims and embodiments are within the scope of the disclosure. 

What is claimed is:
 1. A method for forming a package structure, the method comprising: providing a first substrate having a plurality of contact pads formed thereon; applying a first solder on the plurality of contact pads; attaching a plurality of conductive pillars on the plurality of contact pads by the first solder, the first solder extending between the plurality of contact pads and the plurality of conductive pillars; and after attaching the plurality of conductive pillars, bonding a second substrate to the plurality of conductive pillars with a second solder.
 2. The method of claim 1, wherein the step of attaching the plurality of conductive pillars on the plurality of contact pads by the first solder comprises: providing a support substrate having a plurality of cavities; disposing the plurality of conductive pillars in respective ones of the plurality of cavities; and attaching the plurality of conductive pillars in the cavities to the first solder using a reflow process.
 3. The method of claim 2, wherein disposing the plurality of conductive pillars comprises: disposing the plurality of conductive pillars over a surface of the support substrate; and vibrating the support substrate such that the plurality of conductive pillars fall into the plurality of cavities.
 4. The method of claim 1, wherein the first solder extends through an insulating layer.
 5. The method of claim 4, wherein the plurality of conductive pillars has a width less than a width of the first solder extending through the insulating layer.
 6. The method of claim 4, wherein the plurality of conductive pillars has a width greater than a width of the first solder extending through the insulating layer.
 7. The method of claim 1, wherein the plurality of conductive pillars has a non-uniform width.
 8. The method of claim 1, wherein after bonding the second substrate, sidewalls of the plurality of conductive pillars are completely covered by the first solder and the second solder.
 9. The method of claim 1, wherein after bonding the second substrate, the first solder does not contact the second solder.
 10. The method of claim 1, wherein the first solder completely separates the plurality of conductive pillars from the plurality of contact pads and the second solder completely separates the plurality of conductive pillars from contact pads of the second substrate.
 11. A method for forming a package structure, the method comprising: applying a first conductive material to a first contact pad of a first substrate; after applying the first conductive material to the first contact pad, attaching a conductive pillar to the first conductive material, the conductive pillar being completely separated from the first contact pad by the first conductive material; and attaching a second substrate to the conductive pillar, the second substrate having a second contact pad and a second conductive material, the second conductive material completely separating the conductive pillar from the second contact pad.
 12. The method of claim 11, wherein the first contact pad is partially covered with a first insulating layer, the first conductive material extending through the first insulating layer to the first contact pad.
 13. The method of claim 12, wherein after attaching the second substrate, the first conductive material contacts the second conductive material.
 14. The method of claim 12, wherein a distance between the first contact pad and the conductive pillar is greater than a distance between the first contact pad and an outermost surface of the first insulating layer from the first contact pad along a shortest line between the first contact pad to the second contact pad.
 15. The method of claim 11, prior to attaching the second substrate, forming protective layer around the conductive pillar.
 16. The method of claim 15, wherein the protective layer contacts sidewalls of the conductive pillar.
 17. A method for forming a package structure, comprising: applying a first conductive material to a first contact pad of a first substrate; after applying the first conductive material to the first contact pad, reflowing the first conductive material and attaching a first end of a conductive pillar to the first conductive material; and after reflowing the first conductive material, reflowing a second conductive material attaching a second end of the conductive pillar to a second contact pad of a second substrate, thereby bonding the first substrate to the second substrate, the conductive pillar being spaced apart from the first contact pad and the second contact pad.
 18. The method of claim 17, further comprising, after reflowing the first conductive material and prior to reflowing the second conductive material, forming a protective layer on the first substrate, the conductive pillar protruding through the protective layer.
 19. The method of claim 18, wherein the protective layer completely covers the first conductive material.
 20. The method of claim 19, wherein after reflowing the second conductive material, the second conductive material contacts the protective layer. 